Rotary clamps are known of the type in which linear actuator reciprocating movement is adapted to be translated into angular rotary movement of a clamp arm. The linear actuator may be powered by a fluid motor, and an additional linkage or other transmitting means converts the linear actuator motion to a rotary clamp motion. Normally, when the fluid motor is in a retracted position, the clamp is in a released position, that is, the clamp arm is removed from the work supporting surface, and by means of fluid pressure, the clamp arm is pivotally moved into clamping position to clamp a work piece to a work supporting surface and hold and/or locate the work piece against the work supporting surface.
Various guide and linkage means have been proposed to correctly translate linear reciprocating movement of a piston and piston rod, utilized in the linear actuator, to correctly swing the clamp arm into or out of clamping position, seeking to obtain the highest mechanical advantage which can be utilized within the power stroke of the linear actuator. All of these known mechanisms, more or less, include complex designs of various mechanical components at high manufacturing and assembly cost.
A known design powers a linear actuator along a guide slot provided in a housing of the clamp. The end of the linear actuator is pivotally connected to a linkage member which in turn is pivotally connected to a lever arm of a pivot pin. The linear actuator provides reciprocal linear movement along the guide slot, thus driving the linkage member and converting the linear movement into rotational movement of the pivot pin. A clamp arm is connected to the pivot pin for rotary motion of the clamp arm between a clamped position and an unclamped position.
Typically, such designs try to prevent the over travel of the linkage member to an over-center position wherein the pivot points of the linkage member are at a zero degree angle with respect to one another, in other words, the longitudinal axis of the linkage member is at a position perpendicular to the longitudinal axis of the guide slot. As the linkage angle approaches zero degrees, the linkage force approaches infinity through the relationship P=F.div.tan.alpha. where P=the linkage force, F=the linear actuator force and .alpha.=the linkage angle. Thus, as the linkage member approaches the over-center position, the clamp mechanism experiences ultra-high linkage forces which may cause premature wearing of the linkage mechanism or may cause the linkage mechanism to "freeze" or "lock up". For example, when a clamp arm is adjusted to provide maximum clamping pressure on a work piece at standard factory air pressure, such as 80 p.s.i., any travel to center or slight over-center of the linkage member has been found in most commercial clamps currently available to require a release pressure exceeding the 80 p.s.i. supply line pressure by as much as 20-30 p.s.i. Since this may result in a "lock up" clamp which cannot be released by standard air pressure, such clamps are normally operated with a limited travel of the linear actuator or lever arm to a linkage angle short of 0.degree., i.e. in the order of 8.degree.-4.degree., to assure that supply line pressure will always release the clamp.
Other designs provide for the linkage mechanism to travel to a positive center or slight over-center locking position wherein needle bearings are utilized so that the cylinder pressure required to release the clamp is no greater than the cylinder pressure needed to actuate the clamp to the locking position. Such configurations are capable of producing high clamping forces, but they are also subject to undesirable wear to the internal mechanism created during the passage through the ultra high force over-center position. The result of the wear is the reduction of the clamp forces in which the clamp can produce.
When trying to control the travel of the linkage mechanism, most clamp designs do not consider controlling the clamping force. Applicant's U.S. Pat. No. 4,905,973 prevents the linkage mechanism from reaching the over-center position by having a positive stop on the lever arm engage an internal surface in the housing of the rotary clamp. Others provide a stop to a portion of the linear actuator, and thus, the over-center position may never be reached although wear may create play within the linkage mechanism thereby effecting the clamping force of the clamp arm. Others provide a stop for the clamp arm while allowing the linkage mechanism to near and approach the over-center position.
Although preventing the linkage member from reaching the over-center position may ensure that the supply line pressure will release the clamp, most clamps do, not provide a means by which to manually release the clamp without disassembling the clamp should the power and/or control means fail to operate the clamp when in the clamped position. This is especially so when the internal mechanisms of the rotary clamp are fully disposed within an enclosed housing.
Some clamp designs prevent the linkage member from reaching an over-center position by utilizing a positive stop on the lever arm to engage the housing and limit the clamping position and force of the clamp arm. Since the lever arm is coupled to the linkage assembly, the linkage member is prevented from reaching the over-center position. Due to the number of reciprocating cycles realized by the linkage mechanism during the life of the clamp and due to the large and variable forces that are realized by the linkage mechanism during those cycles, the tolerances or "slop" within the linkage mechanism begin to increase thus allowing the linkage member to slowly approach the over-center position. Thus, it would be desirable to provide a secondary stop to ensure that the linkage member cannot reach the over-center position while still maintaining a consistent clamping force and position over the life of the clamp.
The linkage force and actuation force may also be affected by additional friction and binding created by the linkage mechanism and linear actuator. Often the linear actuator is subject to forces having components that are perpendicular to the line of linear motion. Often side walls are machined within the guide slot of the housing to support the linear actuator against these forces. Such guide slots are difficult and expensive to manufacture thus creating an undesirable situation in industry.